Cancer is one of the leading causes of death in the developed world, with over one million people diagnosed with cancer and 500,000 deaths per year in the United States alone. Overall it is estimated that more than 1 in 3 people will develop some form of cancer during their lifetime. There are more than 200 different types of cancer, four of which—breast, lung, colorectal, and prostate—account for over half of all new cases (Jemal et al., 2003, Cancer J. Clin. 53:5-26).
Folate Receptor 1 (FOLR1), also known as Folate Receptor-alpha or Folate Binding Protein, is an N-glycosylated protein expressed on plasma membrane of cells. FOLR1 has a high affinity for folic acid and for several reduced folic acid derivatives. FOLR1 mediates delivery of the physiological folate, 5-methyltetrahydrofolate, to the interior of cells.
FOLR1 is overexpressed in the vast majority of ovarian cancers, as well as in many uterine, endometrial, pancreatic, renal, lung, and breast cancers, while the expression of FOLR1 on normal tissues is restricted to the apical membrane of epithelial cells in the kidney proximal tubules, alveolar pneumocytes of the lung, bladder, testes, choroid plexus, and thyroid (Weitman S D, et al., Cancer Res 52: 3396-3401 (1992); Antony A C, Annu Rev Nutr 16: 501-521 (1996); Kalli K R, et al. Gynecol Oncol 108: 619-626 (2008)). This expression pattern of FOLR1 makes it a desirable target for FOLR1-directed cancer therapy.
Because ovarian cancer is typically asymptomatic until advanced stage, it is often diagnosed at a late stage and has poor prognosis when treated with currently available procedures, typically chemotherapeutic drugs after surgical de-bulking (von Gruenigen V et al., Cancer 112: 2221-2227 (2008); Ayhan A et al., Am J Obstet Gynecol 196: 81 e81-86 (2007); Harry V N et al., Obstet Gynecol Surv 64: 548-560 (2009)). Thus there is a clear unmet medical need for more effective diagnostics for ovarian cancers.
Some previous assays used to detect shed FOLR1 are not sufficiently specific to FOLR1. For example, some assays do not distinguish between FOLR1 and other folate receptor family members (FOLR2, 3, & 4) or report values for total FBP (Folate Binding Protein). Additionally, some assays require that human samples (e.g., plasma) be pre-treated with a light acid wash step to dissociate folic acid from the receptor. Some assay results may also have inaccuracies due to competitive effects between the antibody therapy and diagnostic antibody. Additionally, many commercially available kits are traditionally unreliable both in their reagents, and in their lot-to-lot stability. Evaluations of these kits have given very mixed results, and are intended for research use only. Many require that the human sample be pre-diluted before analysis to reduce the chance of false positives due to the “matrix effect.” Thus, there is a clear need for highly sensitive and accurate diagnostic assays that can detect a clinically relevant dynamic range of FOLR1 as a companion for FOLR1-based therapies.